Male hamsters will be used in a series of studies designed to provide information about neural control of circadian rhythm and photoperiodic time measurement. (1) Knife cuts or chemical lesions will destroy selected afferent systems of the suprachiasmatic nuclei; (2) Knife cuts will be made around the suprachiasmatic nuclei to sever their efferent connections with adjacent hypothalamus; (3) The two suprachiasmatic nuclei and the associated optic chiasm will be separated along the midline. Locomotion and body temperature rhythms will be continuously monitored and analyzed for the extent of disruption following lesions. Similarly designed experiments will test for the presence or absence of short photoperiod induced gonadal regression following the lesions. Nissl and immunohistochemical stains as well as amino acid autoradiographic techniques will be used to assess the effectiveness of the cuts and lesions. The studies will elucidate neuroanatomical control of two circadian rhythms and permit comparison with neuroanatomical control of photoperiodism.